The present invention relates to a continuous process for the manufacture of fiber-reinforced, hydraulically bound articles. This is accomplished by placing a network and a hydraulically bindable material, on top of two endless conveyors with the conveyors being positioned so that one layer formed on one of the endless conveyors is deposited onto the layer formed on the other conveyor, by use of a reversing roller. Thereafter, the composite structure formed from the two layers is giving the described shape and then hardening the shaped structure. Hydraulically bound material as used herein denotes material bound or hardened by reaction with water.
It is known to employ fibers in hydraulically bound materials to improve the mechanical properties of the article such as the impact strength. The fibers may be of natural original or man-made, inorganic or organic in nature, and may be used in the form of continuous or discontinuous monofilaments or in bundles of filaments. Short pieces of fiber may be mixed with hydraulically bindable material or layers of such fiber pieces may be incorporated in the hydraulically bindable material. However, it is the random orientation of the fibers within the material that is one of the reasons why fiber reinforcement has so far failed to fully satisfy expectations. A drawback inherent in the known way of applying fiber reinforcement to such articles is that part of the fibers do not contribute to the improvement of the properties of the hydraulically bound materials. This is caused not only by their random orientation, but also by their presence in the material in places where they are not needed, e.g. on the neutral axis when bending loads are exerted.
British Pat. No. 118,395 describes a machine for manufacturing plaster slabs in which canvas has been incorporated more or less in the middle of the plaster as a reinforcing element, with the plaster being covered on both sides with a layer of paper or fabric, and, optionally, with additional strips of the fabric applied to the paper or fabric layers so as to ensure smoothness of the cutting plane. The disadvantage inherent in this composition of the plaster slabs is that the fabric network is applied along the axis that is neutral under bending loads. Further, there is no suggestion in this patent concerning the formation of two outer layers in which fiber reinforcement in the form of a network is distributed, with such reinforcing material being located in a position to best assist in absorbing load forces. Further, the machine does not comprise means for removing water by suction or for the application of intermediate or core layers within the composite structure.
French Pat. No. 1,224,611 describes a process and apparatus for manufacturing asbestos-cement sheets, in which several layers of an asbestos-cement slurry are applied, by means of dip rolls, to the undersides of felt bands. The slurry is kept in contact with the felt bands by the application of suction on top of the bands. The layers of slurry formed on the felt bands are united to a single layer by reversing the motion of one of the felt bands, after which the sheets are turned upside down and cut to measure. While it is undesirable to use asbestos fibers, fiber reinforcement is again placed in areas that are neutral with respect to the absorption of load forces, and there is again no suggestion as to the desirability of nor means for applying an intermediate or core layer. Further, the process and apparatus are not suitable for working with fiber reinforcing material in the form of continuous networks of polymeric material, because the hydrophobic nature of such material would repel the cement slurry and the suction force referred to above would be ineffective, if the cement mortar without fibers could at all be applied by means of dip rolls.
U.S. Pat. No. 3,944,698 describes a process for continuously manufacturing gypsum wallboard that includes fiber reinforcing on the outside. The fibers are affixed to a layer of paper and the fibers may be in the form of continuous filaments, arranged in the form of an openwork fiber mat, such as a fishing net or curtain netting, or alternatively a mat composed of short fibers. To a first composite sheet of paper and fibers a layer of gypsum is applied, after which, by reversing the direction of motion, a second composite of paper and fibers is deposited on the layer of gypsum. Subsequently, the gypsum board is given cut to the required length and allowed to harden. The disadvantage of this process is that although a fiber reinforcement is applied to both exterior sides of the gypsum, the resulting structure is not a composite laminate with the outer layers consisting of fiber-reinforced water-hardening material that have undergone the same treatment and are similarly composed. The resulting sandwich-like laminate is identical only at the extreme outside surfaces, as regards the combination of paper and fibers.
Because mesh or network type structures comprised of fibers have been found to represent a considerable advance in improving the properties of hydraulically bound materials, partly because in such structures the fibers are both oriented and well distributed, the present invention is aimed at providing a process for the continuous manufacture of fiber-reinforced materials which avoids the drawbacks of prior processes, but one which better utilizes the relatively expensive reinforcing structures.